1. Field of the Invention
This invention pertains to borehole drilling apparatus and specifically to that part of a drill string known in the industry as a stabilizer.
2. Description of the Prior Art
Stabilizers, sometimes referred to as drill collar stabilizers or as drill stem stabilizers, have been employed in earth boring operations for the petroleum industries to centralize the drill stem in the borehole, usually especially in the drill collar section at a distance of from 100 feet to 1000 feet above the drill bit. The purposes of a stabilizer are to (1) help control hole angle direction, (2) prevent the bit from drifting laterally, which would result in undesirable dog-legs and ledges, and (3) improve bit performance by forcing the bit to centrally rotate about its axis so as to provide substantially equal force loading on all three drill bit cones. In addition, stabilizers also may be used to provide a reaming function for undersized or irregularly shaped boreholes providing the formation is not too hard.
Stabilizers are categorized in the industry as rotating stabilizers and as non-rotating stabilizers. A rotating stabilizer includes wall-contacting members that rotationally track along the wall of the borehole as the drill string is turned. On the other hand, non-rotating stabilizers, one type of which is also referred to as sleeve-type stabilizers, do not rotate as the drill string is turned, its wall-contacting members merely moving around the wall of the borehole as the drill string is rotated and lowered or raised.
The contacting members of a rotating type of stabilizer, which is the type of stabilizer described herein, are subjected to the various forces attendant to the entire drill string, including thrust forces, fretting forces, and the forces applied to the drill string as a result of the drill string manipulations, the conditions of the bore, and the fluid conditions internal and external to the drill string.
Various rotating types of stabilizers include mechanisms for connecting and detaching the wear elements to provide for their replacements. Such connectable and detachable mechanisms include various slot and groove connections, cap screw connections, tapered wedging connections and combinations of the above. However, one of the most popular types of stabilizer of the rotating variety is known as the "welded-blade" stabilizer. Its popularity stems from the fact that there are no connectable mechanisms between the parts other than the permanent welds that affix the wear element, commonly known as the blade, to the drill string member.
Welded blade stabilizers in the prior art are typified by the structures shown in Ortloff et al., U.S. Pat. No. 3,263,274. The wear pads shown in the Ortloff structures are welded to the body of the tool joint; however, it should be recognized that tool joints are typically made of alloy steel which are difficult to weld in reliable fashion, particularly under field environment. That is, welding thereon can only be reliably performed in a controlled environment where the alloy steel is heated and cooled quite slowly and in a clean environmental surroundings. When welding of this type is attempted in conditions other than such a controlled environment, the alloy steel cools too quickly and results in cracking of the weld. Even when welds are made under controlled conditions, some damage is done to the alloy structural member. This damage is accumulative and irreversible. After many replacements are made the lasting damage by itself may be enough to cause the occurrence of cracks.
There are also structures in the prior art, such as shown in Creighton, U.S. Pat. No. 2,288,124, that discloses stabilizer elements held within sleeves which are, in turn, welded to the tubular body. Although there are many different structures shown in Creighton, one of which has elements welded to the surface of the protector body sleeve (facing 29 welded to body 10c in FIG. 8 of Creighton), there is no showing in Creighton of the use of a sleeve which is particularly suited for affixing weldable wear pads thereto.
Therefore, it is a feature of the present invention to provide a stabilizer having wear elements that are reliably affixable by welding under field conditions.
It is another feature of the present invention to provide a stabilizer with a sleeve or other suitable arrangement to permit wear-element affixing to the stabilizer tool body by welding, rather than by clamping, snapping or other releasable means.